With the explosive growth of the Internet, many customers have desired to use the larger bandwidth of a cable television network to connect to the Internet and other computer networks. Cable modems, such as those provided by 3Com Corporation of Santa Clara, Calif., and others offer customers higher-speed connectivity to the Internet, an intranet, local area networks (“LANs”) and other computer networks via cable television networks. These cable modems currently support a data connection to the Internet and other computer networks via a cable television network with a data rate of up to 30+ Mbps which is a much larger data rate than can be supported by a modem used over a standard telephone line.
The physical plant of the data-over-cable system may be divided into physically isolated branches that distribute digital signals from a cable modem termination system (“CMTS”) to geographically distinct groups of cable modems. Typically, each branch carries the same downstream signal that serves all the cable modems, regardless of which branch they are on. By contrast, in the reverse or upstream direction, the cable modems on any given branch transmit to the CMTS with a set of common transmission parameters, which may differ from the upstream transmission parameters for another branch. Thus the transmissions to the CMTS on each branch are independent of the transmissions on the other branches. Typically, the transmissions on different branches do not interfere with each other because the branches are physically isolated from each other.
When a cable modem is connected to a data-over-cable system, it must first gain access to the system by undergoing an initialization procedure before it is permitted to transmit data. A part of the initialization procedure includes a ranging process to determine the appropriate transmission parameters for its data transfer. This process involves sending a ranging message at an appropriate time interval, referred to as the Initial Maintenance interval.
In data-over-cable systems with more than one branch, the Initial Maintenance intervals for ranging on each branch are typically independent of each other. Unfortunately, this independence may cause ranging messages from one branch to interfere with upstream data transmissions on another branch. This interference occurs because an initializing cable modem may not have sufficient information to identify to which branch it belongs. In the most typical arrangement of cable modem transmission parameters, the cable modems on each branch transmit upstream at a common transmission frequency. The common upstream transmission frequency usually does not present a problem since the branches are physically isolated. When a cable modem initializes, however, it examines the downstream transmissions and determines when an Initial Maintenance interval is scheduled to occur. The problem is that the cable modem may accept the Initial Maintenance interval for another branch and thus send out ranging messages at the wrong time for its branch.
These mistimed ranging messages generally interfere with legitimate upstream data transmissions from the other cable modems on the branch. One solution to this problem is to arrange that the upstream transmission frequency for each branch is different. The CMTS is tuned to receive signals from each branch at the frequency appropriate for that branch. Then incorrect ranging signals are at a different frequency from the legitimate data transmissions, do not interfere with the data transmissions, and are rejected by the CMTS. In practice, however, the limited bandwidth for upstream radio frequency propagation does not permit sufficient number of branches to be tuned to different frequencies.
It is, therefore, desirable to reduce interference between ranging messages from initializing network devices on branches of a data-over-cable system and upstream data transmissions.